The present invention relates to the suppression of undesirable radiated emissions and susceptibility in high-speed balanced communication interfaces, and more particularly to an electromagnetic interference (EMI) filter for use in such interfaces.
Modern electronic equipment incorporates high-speed balanced communication interfaces, which are one of the dominant sources of undesirable radiated emission and susceptibility. Radiated emission stems primarily from common mode (CM) currents driven by electronic equipment onto attached communication cables. Electromagnetic interference (EMI) filters, used for suppression of CM currents, normally incorporate capacitors referred to the equipment chassis and CM chokes. In order to eliminate waveform distortions of communication signals, the value of such suppression capacitors, when used in high-speed interfaces (100 BaseT or similar), is limited to a maximum of 10-20 pF. This limitation makes the capacitors less efficient at frequencies below 300 MHz, and imposes the major role of CM rejection onto the CM chokes.
Existing commercially available CM chokes do not provide sufficiently high CM impedance in a wide frequency range. CM chokes produced by windings of pairs of signal wire on ferrite toroid usually have a resonant type of attenuation versus frequency curve, with poor performance outside of a relatively narrow stop-band. Thus, the attenuation curve falls significantly at frequencies both below and above the maximum CM attenuation.
The EMI filters of the present invention are of the lossy type, and are based on the unique absorption properties of glass-coated microwire, starting at frequencies above several MHz and steadily improving up to, and including, microwave frequency bands. Microwires employed in the EMI filters according to the invention have a metal core, typically with a diameter from 1 to 30 micrometers, coated by a thin glass layer. Such microwires may be manufactured by one of several well-known methods, e.g., those disclosed in U.S. Pat. No. 5,240,066 (Gorynin, et al.) and U.S. Pat. No. 5,756,998 (Marks, et al.). These microwires, are applied in the field of electronics, to achieve sensors, transducers, inductive coils, transformers, magnetic shields, devices, etc., as taught by U.S. Pat. No. 6,270,591 (Chiriac, et al.), but they have never been proposed as a CM noise-absorbing element in the construction of EMI filters. The absorption properties of the EMI filters according to the present invention are the result of magnetic loss phenomena in glass-coated advantageously amorphous metal microwires, which exhibit strong dissipation in a broad band of radio and microwave frequencies. FIGS. 1a, 1b demonstrate that microwire magnetic properties, in the form of magnetic permeability (xcexc=xcexcxe2x80x2+jxcexcxe2x80x3) in a signal wire pair, may be achieved when the microwire is wound around the pair in such a way that the microwire is oriented along the magnetic field component produced by the CM currents.
The use of absorptive materials for CM noise suppression in cables is known from U.S. Pat. No. 4,506,235 (Mayer), in which it is noted that xe2x80x9cthe electromagnetic field of the symmetrical (differential) mode is confined between the two conductors while the electromagnetic field of the common mode is absorbed in the magnetic absorptive insulating composite.xe2x80x9d In this way, stronger absorption and attenuation were achieved for the CM currents, as compared with the undesirable attenuation of symmetrical (differential) currents. The same principle of segregation of the CM versus differential mode (DM) current components is employed in the EMI filter of the present invention, but with the following distinguishing features:
1) The xe2x80x9cmagnetic absorptive insulating compositexe2x80x9d claimed in the above-mentioned ""235 patent comprises xe2x80x9ca flexible binder having embedded therein manganese-zinc ferrite particles, having a non-homogenous particulate mix consisting essentially of smaller particles of 10-100 xcexcm and larger particles of 150-300 xcexcm, but wherein said particles are at least as large as the size of the magnetic domain of the ferrite . . . xe2x80x9d In the present invention, the absorbing media is composed of glass-coated microwires.
2) The Mayer invention has for its object xe2x80x9can improved electrical transmission cable with two conductors, protected against electromagnetic interferences (EMI)xe2x80x9d, while the object of the present invention is the provision of miniature EMI filter components, primarily for application inside protected equipment, on printed circuit boards (PCBs), mostly in the vicinity of interface connectors.
3) The Mayer U.S. Pat. Nos. 4,383,225 and 4,301,428 disclose, in general, filter wires and cables comprising an inner conductive wire or multi-conductive wire cable, covered with an outer layer of magnetic shielding. In contrast to the magnetic shielding layer of Mayer, the present invention utilizes a magnetic absorptive layer comprising a glass-coated microwire having a metal core exhibiting unique magnetic properties.
The novel EMI filters of the present invention have the following advantages, gained primarily due to the use of unique glass-coated microwire:
a) exclusive broadband and high CM attenuation characteristics, covering VHF, UHF and microwave frequency bands, substantially exceeding any existing ferrite-based CM chokes or lossy-type EMI filters in performance;
b) low differential-mode loss, up to at least 100 MHz, making the filters applicable on high-speed communication wire pairs; and
c) miniature size and SMD packaging, suitable for automatic placement on a customer""s PCBs.
A broad object of the present invention is to provide a novel signal and/or power PCB-mounted EMI filter, affording high CM attenuation in a wide frequency band, based upon the use of special structures and materials having unique magnetic absorbing properties.
It is another object of the present invention to provide an EMI filter component that achieves high Common Mode (CM) attenuation values in the frequency range from about 10 MHz up to at least 18 GHz, and low attenuation to Differential Mode (DM) signals.
The invention therefore provides an electromagnetic interference filter, comprising a core having at least one electrically conductive signal or power-insulated lead; at least one first layer surrounding the lead, made of glass-coated microwire serving as magnetic absorbent material; a tubular conductive material surrounding the first layer, and a substrate on which the core is mounted, the substrate being configured as a planar body having a top, a bottom and side surfaces, portions of the top and bottom surfaces being covered with electrically conductive material serving as signal and ground terminals and making electrical contact with the tubular conductive material of the core.
The invention will now be described in connection with certain preferred embodiments with reference to the following illustrative figures, so that it may be more fully understood.
With specific reference now to the figures in detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of the preferred embodiments of the present invention only, and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for a fundamental understanding of the invention, the description taken with the drawings making apparent to those skilled in the art how the several forms of the invention may be embodied in practice.